It is well known to employ resilient elastomeric gaskets in manhole assemblies for providing water-tight seals. For example, U.S. Pat. No. 4,159,829 issued Jul. 3, 1979, and assigned to the assignee of the present invention, discloses a resilient, elastomeric gasket which is preferably extruded and has a cross-section which is designed to provide a substantially T-shaped portion for embedment into the opening of a manhole base and which has an integral, substantially pear-shaped portion extending radially inward from said anchoring portion for yieldable engagement with the outer periphery of a pipe inserted into said opening. The advent of the gasket disclosed in U.S. Pat. No. 4,159,829 showed that it was possible to form a disc-like washer from a linear extrusion by providing a pear-shaped pipe engaging portion integrally joined with a T-shaped anchoring portion, the ends of the extruded member being curved into the shape of a right cylinder after having being cut to the desired length, the ends of said linear extrusion being joined to each other such as by vulcanizing. The aforesaid gasket lends itself well to assembly within a manhole base through the use of a mold assembly of the type described in U.S. Pat. No. 3,796,406, assigned to the assignee of the present invention. The aforesaid mold assembly comprises first and second frustoconical-shaped mold members which, when joined, define an inwardly directed peripheral groove which encloses the apex of the gasket pipe engaging portion, while the outward radial portion of the gasket is fully exposed, facilitating firm embedment of the anchoring portion within the cast material forming the manhole base.
The gasket of U.S. Pat. No. 4,159,829 has been found to provide a more effective water-tight seal than gaskets molded as an apertured disk or washer. The gasket also provides good resistance to compression near the bottom portion of the manhole base opening while providing sufficient capacity for expansion of the gasket near the top portion of the manhole base opening. Although the gasket very tightly hugs the pipe which it surrounds, the range of pipe size which the gasket is capable of accommodating is limited.